1. Field of the Invention
This invention relates to electromagnetic conversion devices involving electromagnetic coupling. More particularly, this invention relates to electromagnetic actuators, transducers, measuring, indicating and monitoring devices.
2 Prior Art
Various electromagnetic conversion devices are described in the prior art, such as: the permanent-magnet, moving-coil (D'Arsonval) mechanism; the moving-magnet, fixed-coil mechanism; the moving-iron, fixed-coil mechanism, the moving-coil, fixed coil mechanism. The most pertinent involves tbe D'Arsonval movement. An exemplary description of the D'Arsonval movement is found in the Standard Handbook for Electrical Engineers, edited by Donald G. Fink and H. Wayne Beaty (11th Edition, 1978).
The D'Arsonval movement comprises a wound coil of wire ("coil") that is suspended between bearings in a magnetic field and is therefore free to rotate ("moving coil"). The magnetic field is produced by a permanent magnet which surrounds the moving coil. An air gap is maintained between the magnet pole pieces and the moving coil. Usually, some form of restraint such as a spring is attached to the moving coil. Tbe D'Arsonval mechanism operates by having the current, which flows in the coil, react to the magnetic field that surrounds the movable coil and that reaction is counter-balanced by the force from the spring. Tbe force on the moving coil, caused by the magnetic field about the coil from the electric current flowing through its conductors and the magnetic field of the gap, causes the coil to rotate to the point where the force on the coil due to the magnetic interaction is equal and opposite to the force on the coil from the spring.
While the D'Arsonval mechanism and the other aforementioned mechanisms have been widely accepeted, the springs required for their operation have caused various problems. In many applications, the output response of the mechanism should be proportional to the electrical input. In an ammeter, for example, the deflection of the wound coil should be proportional to the input current in the winding. Thus, the force associated with the spring should be a linear function of the displacement, but in fact it is not exactly linear because the spring coefficient is not totally linear over the range of the displacement. Thus, tbe output response of the D'Arsonoval mecbanism is linear only for a limited angular displacement of the wound coil.
The metallic springs which are a part of the mechanisms identified above, are temperature sensitive in that the spring constant varies with temperature. Another disadvantage associated with the spring is the problem of overcoming hysteresis associated with the wound metallic member. Finally, it is simpler to calibrate a movement whicb does not require a spring.